1. Field of the Invention
The present invention generally relates to a wireless communication system. More particularly, the present invention relates to an apparatus and method for transmitting and receiving signals in a Multiple-Input Multiple-Output (MIMO) communication system.
2. Description of the Related Art
In general, a transmission signal experiences more severe distortion in a wireless environment than in a wired environment due to various factors, such as multipath fading, attenuation, time-variant noise, interference, etc. Multipath fading is closely related to reflection from terrestrial objects or movement of a user. A distorted version of a signal transmitted on a fading channel arrives at a receiver, thus degrading the whole system performance. The fading phenomenon is one of the critical factors impeding high-speed data communications in wireless environments. Therefore, loss of a radio channel caused by fading and interference between users is a challenging issue to tackle in order to implement high-speed data transmission in wireless environments.
To overcome fading, spatial diversity-based transmission schemes have been proposed and are under active research. Spatial diversity techniques include Transmit (Tx) antenna diversity using multiple Tx antennas and Receive (Rx) antenna diversity using multiple Rx antennas. A system for achieving spatial diversity using multiple Tx/Rx antennas is called a MIMO system.
A MIMO system decides what data to send through each of multiple Tx antennas by space-time coding, and Rx antennas each receive signals from the Tx antennas and perform space-time decoding.
Compared to space-time coding schemes, spatial diversity schemes send different data streams through different Tx antennas, thereby enabling high-speed data transmission and achieving a multiplexing gain as well.
In spatial diversity schemes, a receiver decodes a received signal by joint or separate detection. A joint detection process takes into account a signal transmitted by one Tx antenna as well as signals transmitted by other Tx antennas.
Due to this feature, Maximum Likelihood (ML) decoding is known as an optimal decoding process for a spatial diversity-MIMO system. ML decoding offers a diversity order equal to a number of Rx antennas irrespective of a number of Tx antennas. ML decoding outperforms other decoding processes, such as Minimum Mean Square Error (MMSE) or Zero Forcing (ZF), in terms of Signal-to-Noise Ratio (SNR). An SNR gain increases in proportion to the number of Tx antennas.
However, a MIMO system using such a multiplexing scheme can achieve a multiplexing gain only without a Tx diversity effect. In contrast, a space time coding scheme can improve reception reliability with a diversity gain but is not favorable for high-speed data transmission. Accordingly, a need exists for developing a Full Diversity Full Rate (FDFR) scheme that achieves a diversity gain with a minimized decrease in data rate.